Large field, high resolution radiant energy detection system



c. w. HARRIS ET AL 3,180,206

4 Sheets-Sheet 1 molar-Om LARGE FIELD, HIGH RESOLUTION RADIANT ENERGYDETECTION SYSTEM April 27, 1965 Filed Aug. 9, 1960 moi: JE..:o m N Qkk335w wuu= 323 m ouutam 30E N n 3.23:3 O m. S m H U R W W N S E 6 W l W Bw w uufimam N32. CP 455325. 2 N. T o N ZOE-5.0m mom m2 m2 RN 9* 3 mP5016 55.50 ozawmoomm moZmhE .l zoF N =5 um N N N mm \IN mm N ON mm 26-.E WW4 223m 2200 50 3:91am o mcovcoo D I A ril 27, 1965 c. w. HARRIS ETAL 3,180,206

LARGE FIELD, HIGH RESOLUTION RADIANT ENERGY DETECTION SYSTEM Filed Aug.9, 1960 4 Sheets-Sheet 2 Fig. 3.

Clyde W. Harris, Paul S. Sugino,

INVENTORS.

AGE/VT.

Aprii 27, 1965 c w, ms AL 3,180,206

LARGE FIELD, HIGH RESOLUTION RADIANT ENERGY DETECTION SYSTEM Filed Aug.9, 1960 4 Sheets-Sheet 5 Clyde W. Harris, Poul S. Sugino,

INVENTORS.

AGE/VT.

April 27, 1965 c. w. HARRIS ET AL 3,180,206

LARGE FIELD, HIGH RESOLUTION RADIANT ENERGY DETECTION SYSTEM Filed Aug.9. 1960 4 Sheets-Sheet 4 Clyde W. Harris,

Paul S. Sugino,

INVENTORS.

United States Patent LARGE FEELB, iii-Gil RESULWZGN RADHAN'E ENERGYDETECTIQN SYSTEM Clyde W. Harris and Paul S. Sugino, Santa Barbara,Calitl,

assignors to Hughes Aircraft (Iompany, Quiver titty, Calii, acorporation of Delaware Filed Aug. 9, 196%, Ser. No. 43,

It) (Balms. (Ci. 88-1) The present invention relates to apparatus foretecting the presence of radiant energy and, more particularly, to anopitcal detection system for detecting radiant energy such as that theinfrared spectrum, the detection system having a wide field of view andproviding high resolution yet utilizing a simple objective lens.

Heretofore, radiant energy detection systems have utilized aninterruption reticle or episcotister having a fiat interruption surface.Consequently, the objective lens has heretofore been corrected toprovide good resolution and definition of object images over the flatreticle surface. It is well known in the lens art that it requires thehighest degree of design and manufacturing skill to achieve highresolution over a fiat image field for a moderately large field of view.Attempts to increase the effective aperture s ze of the objective lenswithout sacrificing the definition of the image or the area of the imagefield has resulted in objective lenses of complex construction uitlizingspecial types of lens materials. The size of the entrance pupil oraperture is a large factor in determining system ensitivity and thusshould be as large as possible. A multiple lens must possess opticalelements whose diameters are greater than the diameter of the entrancepupil, thus causing lenses for focusing a wide field of view to beextremely large and extremely expensive. Furthermore, complex lenses donot have high transparency because of the loss of transmitted energythrough the large numberof relatively thick lens elements.

it has heretofore been considered diliicult to construct radiant energydetection systems having a wide field of view for the reasons discussedabove. A field of view on the order of has been considered to be anextremely large field of view and has necessitated the use of large,expensive complex lenses. Fields of view larger than approximately 15have been considered impractical to attain in radiant energy detectionsystems.

Accordingly, it is an object of the present invention to provide radiantenergy detecting apparatus that has a high resolution and wide field ofView.

Another object of the invention is the provision of a radiant energydetector utilizing a simple objective ens of a size substantiallyidentical to that of the entrance pupil and yet providing a Wide fieldof view and high resolution.

in accordance with these and other objects or" the invention, a simpleconverging objective lens of a size substantially equal to that of theentrance pupil focuses radiant energy admitted by the entrance pupilintoa concave image surface A transparent rcticle is disposed at the imagelocation and has a concave surface substantially conforming to theconcave image surfac The concave surface of the reticle is provided withopaque areas forming an interruption pattern and apparatus is arrangedto provide relative motion between the optical system and the reticle.In oneembodimcnt of the device, the concave reticle surface conforms tothe tangential image surface of the lens and the interruption pattern isin the form of a spiral. In a second embodiment of the invention, theconcave reticle surface conforms to the sagittal image surrace of thelens and the int rruption pattern is in the form of radial spokes.

The following specification and the accompanying Eidhifih Patented Apr.27, 1965 drawings respectively describe and illustrate exemplifiestionsof the present invention. Consideration of the specification and thedrawings will provide a complete understanding of the invention,including the novel features and objects thereof. Like referencecharacters are used to designate like parts throughoutthe figures of thedrawings. 7

PEG. 1 is a diagram illustrating the image surfaces formed by a simpleconverging lens;

FIG. 1A is a diagram illustrating the shape of the images formed in thetangential image surface of the sim ple converging lens of FIG. 1;

PEG. 1B is a diagram illustrating the shape of the images formed in thesagittal image surface of the simple converging lens of FIG. 1;

PEG. 2 is a diagram illustrating an embodiment of a radiant energydetection system in accordance with the present invention;

FIG. 3 illustrates an interruption pattern that may be used on thereticle of the radiant energy detection system of PEG. 2 for atangential image surface;

FIG. 4 illustrates an interruption pattern that may be used on thesurface of the reticle of the radiant energy detecton system of FIG. 2for a sagittal image surface;

PEG. 5 illustrates an arrangement for nutating an image focused by asimple converging lens on a stationary reticle;

PEG. 6 illustrates a differentembodiment of a radiant energy detectionsystem in accordance with the invention; and

PEG. 7 illustrates a reticle pattern that may be used in the radiantenergy detection system of P16. 6'with a tangential image .trface. I

The present invention is embodied in a radiant energy detection system,illustrated in FIG. 2, which includes an entrance pupil 2h that admitsradiant energy to an objective lens 21. intercepted radiant energy isfocused by the lens Zil on an interruption reticle that is rotated byrotation means Radiant energy passing through the reticle .23 isconcentrated by a condensing lens 25 on a radiant energy detector Anelectrical signal produced by the detector 26 is applied to a signalprocessing circuit 27 where it is amplified and applied to a utilizationcircuit 28 for display.

The aperture 2% in the embodiment of the invention depicted in FIG. 2 isan opening in a member that may be one wall of a case enclosing theradiant energy detection system. However, the aperture 29 need not be aseparate member inasmuch as the lens 21 only focuses energy passingthrough it and therefore serves as an aperture.

The objective lens 21 is, in the present example, a thin, concavo-convexelement but may be any converging optical element. The lens 21 isdisposed adjacent to the entrance aperture 2% and has an optical axis 22extending through the center of the aperture 20. The objective lens 21is substantialy the same size as the entrance aperture 2% and hasspherical surfaces. The objective lens 21 may be made of any materialthat is transparent to the radiant energy of interest'and has a suitableindex of refraction. For intercepted radiant energy in the infraredspectrum, the objective lens 21 may be made of sapphire, for example.Lenses for use in the infrared spectrum may be opaque to visible light.Although the present invention is describedv/ith reference to operationwith radiant energy in the infrared portion of the radiant energyspectrum, it is to be expressly understood that the apparatus may beeasily adapted for use with radiant energy in any part of the radiantenergy spectrum as in the visible or ultraviolet portiomtor example.This be done by proper selection of the lens material and of the radiantenergy detector as, in accordance with Well-known principles.

A simple lens characteristically transforms point sources of energy inthe field of view into lines lying in two distinct concave imagesurfaces. The first is the primary or tangential image surface where theimage lines are arcs centered on the optical axis of the lens Whoselengths increase with distance from the axis. The second is thesecondary or sagittal image surface Where the images are lines radiatingfrom the axis whose lengths also increase with distance from the axis.These image lines are normally quiet narrow, that is, sharply defined,out to large field angles.

In FIG. 1, a simple lens 10, which may be a single converging thinmeniscus lens, as shown, or any other simple lens such as a doublet,focuses intercepted radiant energy into a primary or tangential imagesurface 12 and a secondary or sagittal image surface 13. Both of theseimage surfaces 12 and 13 are concave and form paraboloidal surfaces thatcross the optical axis 11 of the lens at the focal point. The tangentialand sagittal image surfaces 12 and 13 are both concave toward the lens10, the tangential image surface 12 curving closer to the lens 10.

Referring to FIGS. 1A and 1B, an object point on the optical axis 11forms a sharply defined image point in both the tangential and sagittalimage: surfaces 12 and 13. However, an object point off the optical axis11 forms, in the tangential image surface 12, a curved image line 14(FIG. 1A) that is an arc of a circle concentric with the optical axis11. The length of the curved image line 14 increases with the distanceof the object point from the optical axis 11. Another object pointfarther from the optical axis 11 forms a longer curved image line 15 ata greater distance from the optical axis 11.

In the sagittal image surface 13, an object point off the axis 11 formsa straight radial image line 16 (FIG. 1B) extending radially outwardfrom the optical axis 11. The length of the radial image line 16increases with the distance that the object point is from the opticalaxis 11. A second object point farther from the optical axis 11 producesa longer radial image line 17 at a greater distance from the opticalaxis 11. Thus, an object point off the optical axis 11 of the simplelens 10 produces separate and distinct image lines in the tangential andsagittal image surfaces 12 and 13. Image lines formed in the tangentialand sagittal image surfaces 12 and 13 are sharply defined but would beout of focus on a flat imaging screen passing through the focal point ofthe lens 10. Thus when viewed on a flat imaging surface, theout-of-focus image appears as a blurred image point corresponding to theobject point. This property of a simple converging lens, namely, that offocusing the image into sharply defined lines in concave tangential andsagittal image surfaces is generally considered to be an impediment, butis utilized to advantage in the radiant energy detection system of thepresent invention.

A concave reticle 23 is disposed with its center on the optical axis 22at the focal point of the objective lens 21. The reticle 23 is made of amaterial that is transparent to the radiant energy of interest and inthis example, is curved to conform to the tangential image surface ofthe objective lens 21. The curvature of the tangential image surface ofthe objective lens 21 may either be calculated from the lens equationsor may be determined by ray tracing, as is well known. It may sometimesbe satisfactory if the reticle 23 has a spherical surface that onlyapproximates the paraboloidal curvature of the tangential image surface.One of the concave surfaces of the reticle 23 is provided with opaqueareas that form an interruption pattern, as will be describedhereinafter.

The reticle 23 is rotatably mounted in the radiant energy detectionapparatus for rotation around the optical axis 22 as an axis ofrotation. Means 24, such as an electric motor geared to the reticle 23,is provided for rotation of the reticle 23. A condensing lens 25 isdisposed adjacent the reticle 23 for concentrating radiant energy thatpasses through the reticle 23 onto a radiant energy detector 26 that isdisposed on the optical axis 22. The field of view of the radiant energydetection system is made relatively large, 30 or 40 degrees, forexample, by proper selection of the relative size of the aperture 20,size and focal length of the objective lens 21, size of the reticle 23,size and focal length of the condensing lens 25, and size of thedetector 26.

The radiant energy detector 26 may be, for example, a lead sulfide cell.The output of the detector 26 is electrically connected to a signalprocessing circuit 27 that may be an AC. amplifier but which may includeother circuitry, if desired. The output of the signal processing circuit27 is electrically connected to a utilization circuit 28, which may bean oscilloscope, for displaying signals developed by the radiant energydetection system.

As previously mentioned, the reticle 23 of the radiant energy detectionsystem of FIG. 2 is curved to conform to the concave tangential imagesurface of the objective lens 21. One of the concave surfaces of thereticle 23 is provided with an opaque area 30 (FIG. 3) that extends fromthe center of the reticle 23 to the edge thereof in a spiral form. Thespiral may be Archirnedean, that is, a spiral defined by a point movingwith uniform velocity in a stationary straight line across the reticle23 while the reticle 23 rotates with a constant angular velocity.

The opaque area 30 may be applied to the reticle 23 by various methods.One process that has been found satisfactory is a photographic processin which the surface of the reticle 23 to which the spiral is to beapplied is first silvered and then coated with a photo-resist material.An image of a spiral pattern is focused on the surface of the reticle23, after which the reticle 23 is placed in an etchant bath whereportions of the silvered area are etched away to leave the opaque area30 (FIG. 3).

The opaque area 30 in the form of a spiral is intertwined with atransparent area 31 of the reticle 23; also in the form of a spiral. Thewidth of the lines forming the opaque area 30 and the transparent area31 is made to be approximately the size of the image of objects to belocated. This will depend on the size of the objects, the distance ofthe objects, the magnification of the objective lens 21 and theresolution of the lens 21.

In operation, radiant energy is admitted to the radiant energy detectionsystem through the entrance aperture 20 (FIG. 2). The admitted radiantenergy is focused by the objective lens 21 on the tangential imagesurface of the reticle 23. The means 24 rotates the reticle 23 about theoptical axis 22. Background energy, as from a bright sky or largeobjects such as clouds, is not interrupted by the reticle 23 becausethis radiant energy falls on a large area of the reticle 23.Consequently, a portion of the background radiant energy passes throughthe transparent area 31 at all times regardless of rotation of thereticle 23 and no periodic signal appears at the output of the detector26 in response to the background radiant energy.

Objects to be located form a curved line image (FIG. 1A) on the reticle23 having a width comparable to the width of the lines forming theopaque and transparent areas 30 and 31 of the interruption pattern. Asthe reticle 23 rotates, the curved line image is alternately on theopaque area 30 and the transparent area 31. Accordingly, the radiantenergy from the object to be located is periodically interrupted. Onlywhen the object is exactly on the optical axis 22 will periodicinterruption fail to occur. If the spiral is Archirnedean, theinterruption period will be constant'regardless of the position of theimage on the surface of the reticle 23. The detector 26 produces aperiodic signal in response to the radiant energy from the object to belocated. The periodic signal is amplified in the signal processingcircuit 27 and is applied to the utilization circuit 28 which indicatesis that an object of a size comparable to the opaque and transparentareas 3% and 31 of the interruption pattern is in the field of view ofthe radiant energy detection system.

Many modifications to the system may be made in accordance with theinvention. The opaque area St) on the reticle 23 may be a multiplespiral, that is, one having many arms. The spiral may be of a form otherthan Archimedean. The spiral may have different numbers of arms inditferent concentric zones of the reticle 23. This arrangement yieldsradial position information about the object to be located because theinterruption period is different when the image is in different zones.

If desired, the reticle 23 may be curved to conform to the sagittalimage surface of the lens 21, rather than the tangential image surface.In this case, the images are radial lines as shown in PEG. 1B and theinterruption pattern on the reticle 23 is a radial spoke pattern ofsectors that are alternating opaque and transparent areas 39 and 31(FIG. 4).

For some purposes it is desirable to have the reticle 23 stationarywhile the image of an object to be located is mover or nutated over thesurface of the reticle 23 in a circular or other form of path. This maybe accomplished as shown in FIG. 5, for example. An objective lens 49focuses the intercepted radiation onto a mirror 41 disposed at an angleto the lens 40. The mirror 41 may be, for example, at an angle of 45with respect to the lens 4- 3 and the center point of the mirror 41 ison the optical axis 42 of the lens 49. The reticle 23 is dis' posed toreceive the reflected energy from the mirror 41 and is curved to conformto the tangential or sagittal image surface. The reticle 23 is heldstationary while the mirror 41 is movably mounted so that motion of themirror 41 about its center point will cause the image of.

an object to move or nutate over the surface of the reticle 23. Meansfor rotation 43 is operatively coupled to the center of the mirror 41 atan angle 44 to a line perpen-' dicular to the reflecting face thereof tomove the mirror 41 about its center point.

FIG. 6 shows an embodiment of a radiant energy detec tion system inaccordance with the invention that has been found to be satisfactory fordetecting a flash of radiant energy in the infrared spectrum. Thisembodiment is compact and yet provides a wide field of view with highresolution of images. A case 5t which serves as a shield for the opticalsystem is provided with an entrance aperture 51 adjacent to which isdisposed two lenses S2 and 53, together forming a doublet. Theseobjective lenses 52 and 53 are approximately the same size as theentrance aperture 51. Bafiles 54 are provided inside the case 56 toprevent internal reflection of the radiant energy from the inner Wallsof the case 54 A cylindrical support 55 is rotatably mounted inside thecase 5% concentric with the optical axis of the lenses 52, 53 by meansof ball bearings as. At the end of the. cylindrical support 55 adjacentthe lenses 52, 53, a reticle 57 is provided that is curved to conform tothe tangential image surface of the lenses S2, 53.

A first condensing lens 58 is provided inside the support 55 adjacentthe reticle 57. A ring gear 60 encircles the outside or" the support 55and engages a spur gear 61 driven by a motor 62. A second condensinglens 63 forms one end of a hermetically sealed housing 64. This housing64 is embedded in a sealing compound 5 at the end of the case 5% Thefirst and second condensing lenses 5% and 63 concentrate radiant energypassing through the reticle 57 onto a detector 66 disposed within thehousing 6-4. The detector 56 is of the so-called immersion type andcomprises a lead selenide cell 67 immersed in a strontium titanate orsilicon immersion lens or button 68 having a high index of refraction,permitting use of a small cell 67. The detector 65 is mounted on the endof a glass chamber 7t extending into the housing 64. The inside of thischamber '79 is filled with a cold fluid such as liquid nitrogen, forexample, by means of s a tube 71 extending into the interior of thechamber '70 through the insulating compound 65. The interior of thehousing 64 is sealed and evacuated and forms a Dewar flask. Theelectrical connections to the detector 66 are brought out of the housing64 by means of metal pins 72 sealed in the glass forming the chamber'70.

Referring to FIG. 7, the pattern on the reticle 57 is a spiral havingmany arms and having periodic reversals of direction. This spiralpattern is finely divided and is approximately three inch s indiameter'over all. The objective lenses 52 and 53 focus the image of anobject to be located onto the reticle 57 with sufficient definition thatit is interrupted by thepattern shown in FIG. 7. Other objects in thefield of view of the radiant energy detection system larger than theobject to be located produce an image on the reticle 57 that is toodiffuse to be modulated by the pattern shown in FIG. 7. The reticle isrotated by means of the motor 62 driving the support 55 through the spurgear di and the ring gear 6%.

Thus, there has been described a radiant energy detection system that iscompact in size and utilizes a simple objective lens and yet provideshigh image resolution over a large field of view.

What is claimed is: r

1. A radiant energy detection system comprising: an optical systemhaving elements along an optical axis thereof including an objectivelens providing astigmatically separated tangential and sagittal imagesurfaces, and a transparent reticle having a concave surface. transverseto said optical axis and including opaque areas on which an image of aradiant energy source is focused, said concave reticle surface beingpositioned at and occupying one of said astigmatically separated imagesurfaces, said reticle being rotatably mounted, means operativelycoupled to said reticle for providing periodic rotation of said reticleabout said optical axis, and a radiant energy detector disposed tointercept radiant energy passing through said reticle.

2. A device for periodically interrupting radiant energy focused alongan optical axis into astigmatically separated tangential and sagittalconcave image surfaces by a converging objective lens, said devicecomprising: a reticle rotatably disposed at the image location andhaving a transparent concave surface transverse to the optical axis andsubstantially conforming to, positioned at and occupying one of saidimage surfaces, the concave surface of said reticle being provided withopaque areas defining a regular pattern; and means coupled to saidreticle for rotating said reticle about saidoptical axis to causeperiodic interruption of the focused radiant energy by the opaque areason the surface of said reticle.

3. A device for periodically interrupting radiant energy focused alongan optical axis into astigmatically separated concave tangential andsagittal image surfaces by a converging objective lens, said devicecomprising: a reticle rotatably disposed at the tangential image surfacelocation and having a concave surface transverse to said optical axisandsubstantially conforming to, positioned at and occupying the tangentialimage surface, the concave surface of said reticle being provided withan opaque spiral pattern; and means coupled to said reticle for rotatingsaid reticle about said optical axis to cause periodic interruption ofthe focused radiant energy by the opaque spiral pattern on the surfaceof said reticle.

4. A device for periodically interrupting radiant energy focused alongan optical axis into astigmatically separated concave tangential andsagittal image surfaces by a converging objective lens, said devicecomprising: a reticle rotatably disposed at the sagittal image surfacelocation and having a concave surface transverse to the optical axis andsubstantially conforming to, positioned at and occupying the sagittalimage surface, the concave surface of said reticle being provided withan'opaque radial spoke pattern; and means coupled to said reticle forrotating said reticle about said optical axis to cause 7 periodicinterruption of the focused radiant energy by the opaque radial spokepattern on the surface of said reticle.

5. A device for detecting the presence of radiant energy comprising:means defining an entrance pupil having a large aperture for admittingradiant energy within the field of view of said pupil, a convergingobjective lens disposed adjacent said entrance pupil and beingsubstantially the same size as said aperture for focusing the admittedradiant energy into astigmatically separated concave tangential andsagittal image surfaces, said objective lens having an optical axis; atransparent reticle rotatably disposed at the image location and havinga concave surface transverse to said optical axis and sub.- stantiallyconforming to, positioned at and occupying one of the image surfaces,the concave surfaces of said reticle being provided with opaque areasdefining a regular pattern; means coupled to said reticle for rotatingsaid reticle about said optical axis to cause periodic interruption ofthe focused radiant energy by the opaque areas on the surface of saidreticle; and a radiant energy detector disposed to intercept theinterrupted radiant energy and produce an electrical signal in responsethereto.

6. An optical radiant energy detector having a large field of View andhigh resolution comprising: means defining an entrance pupil foradmitting radiant energy from a source within the field of view of thepupil; a converging objective lens disposed adjacent said entrance pupiland being substantially the same size as said entrance pupil forfocusing the admitted radiant energy to form astigmatically separatedconcave tangential and sagittal image surfaces, off-axis object pointsforming elongated are images in the tangential image surface, saidobjective lens having an optical axis; a rotatable reticle disposed atthe tangential image surface location and having a concave surfacetransverse to said optical axis and substantially matching and occupyingthe tangential image surface, the concave surface of said reticle beingprovided with an opaque spiral pattern; means coupled to said reticlefor rotating said reticle about said optical axis to cause periodicinterruption of the focused radiant energy by the opaque spiral patternon the surface of said reticle; and a radiant energy detector disposedto intercept the interrupted radiant energy and produce an electricalsignal in response thereto.

7. An optical radiant energy detector having a large field of view andhigh resolution comprising: means'defining an entrance pupil foradmitting radiant energy from a source within the field of view of thepupil; an astigmatic objective lens disposed adjacent said entrancepupil and being substantially the same size as said entrance pupil forfocusing the admitted radiant energy to form astigmatically separatedcurving tangential and sagittal image surfaces, off-axis object pointsforming elonagted radial images in the sagittal image surface, saidobjective lens having an optical axis; a rotatable reticle disposed atthe sagittal image location and having a curved surface transverse tosaid optical axis and substantially matching and occupying the sagittalimage surface, the curved surface of said reticle being provided with-anopaque radial spoke pattern; means coupled to said reticle for rotatingsaid reticle about said optical axis to cause periodic interruption ofthe focused radiant energy by the opaque radial spoke pattern on thesurface of said reticle; a radiant energy detector disposed to interceptthe interrupted radiant energy and produce an electrical signal inresponse thereto; a condensing lens disposed between said reticle andsaid radiant energy detector for concentrating the interrupted radiantenergy on said detector; and a utilization circuit electrically coupledto said radiant energy detector.

8. An opical radiant energy detector having a large 8 field of view andhigh resolution comprising: means defining an entrance pupil foradmitting radiant energy from a source within the field of view of thepupil; an astigmatic objective lens disposed adjacent the entrance pupiland being substantially the same size as the entrance pupil for focusingthe admitted radiant energy to form astigmatically separated curvingtangential and sagittal image surfaces, off-axis object points formingelongated are images in the tangential image surface, said objectivelens having an optical axis; a rotatable reticle disposed at thetangential image location and having a curved surface transverse to saidoptical axis and substantially matching and occupying the tangentialimage surface, the curved surface of the reticle being provided with anopaque spiral pattern; means coupled to the reticle for rotating saidreticle about said optical axis to cause periodic interruption of thefocused radiant energy by the opaque spiral pattern on the surface ofthe reticle; a radiant energy detector disposed to intercept theinterrupted radiant energy and produce an electrical signal in responsethereto; a condensing lens disposed between'the reticle and the radiantenergy detector for concentrating the interruped radiant energy on thedetector; and a utilization circuit electrically coupled to the radiantenergy detector.

9. In a radiant energy detection system having astigmatic objective lensmeans for focusing radiant energy at astigmatically separated sagittaland tangental image surfaces of revolution along an optical axis,including a reticle, said reticle comprising: a member rotatably mountedin said system at a selected one of said image surfaces for rotationabout an axis of said lens means, said member having a curved surface ofrevolution corresponding in curvature to said selected one of said imagesurfaces, and disposed in registry therewith transverse to said opticalaxis, said member having alternately opaque and transparent portions ofa predetermined size and shape, whereby radiant energy focused on saidmember only passes through said transparent portions of saidmember, andwhereby radiant energy forming an image on said member of apredetermined size and shape is periodically interrupted by rotation ofsaid member aboutsaid optical axis.

10. A radiantenergy detection system comprising: an optical systemincluding an objective. lens having an optical axis and providingastigmatically separated tangential and sagittal image surfaces, amirror, and a stationary transparent reticle having a concave surfaceincluding opaque areas, the reflecting surface of said mirror beingtransverse to said optical axis at less than a right angle, said concavereticle surface being positioned to intercept radiant energy fromsaid'objective lens reflected from said. mirror and occupying one ofsaid astigmatically separated image surfaces, said mirror beingrotatably mounted, means operatively coupled to said mirror forproviding periodic rotation of said mirror about an axis displaced fromthe normal to the reflecting surface of the mirror by a'predeterminedangle to cause nutation of images on said concave reticle surface, and aradiant energy detector disposed to intercept radiant energy passingthrough said reticle.

References Cited by the Examiner UNITED STATES I PATENTS 2,016,780 10/35Hartinger 881 2,725,781 12/55 Banker 881 2,758,502 8/56 Scott et al88-14 2,931,912 4/60 Macleish 881 2,957,384 10/60 Raninen 881 2,981,8434/61 Hansen 250-203 JEWELL H. PEDERSEN, Primary Examiner.

EMIL G. ANDERSON, Examiner.

1. A RADIANT ENERGY DETECTION SYSTEM COMPRISING: AN OPTICAL SYSTEMHAVING ELEMENTS ALONG AN OPTICAL AXIS THEREOF INCLUDING AN OBJECTIVELENS PROVIDING ASTIMATICALLY SEPARATED TANGENTIAL AND SAGITTAL IMAGESURFACES, AND A TRANSPARENT RETICLE HAVING A CONCAVE SURFACES, AND A TOSAID OPTICAL AXIS AND INCLUDING OPAQUA AREAS ON WHICH AN IMAGE OF ARADIANT ENERGY SOURCE IS FOCUSED, SAID CONCAVE RETICLE SURFACE BEINGPOSITIONED AT AND OCCUPYING ONE OF SAID ASTIMATICALLY SEPARATED IMAGESURFACES, SAID RETICLE BEING ROTATABLY MOUNTED, MEANS OPERATIVELYCOUPLED TO SAID RETICLE FOR PROVIDING PERIODIC ROTATION OF SAID RETICLEABOUT SAID OPTICAL AXIS, AND A RADIANT ENERGY DETECTOR DISPOSED TOINTERCEPT RADIANT ENERGY PASSING THROUGH SAID RETICLE.